The present invention relates to high density foam rolls suitable, for example, for transporting and applying toner in an image-forming apparatus such as an electrophotographic printer. Additionally, the invention relates to polyurethane foam toner adder rollers exhibiting high density and stiffness, and to methods for applying toner in print media applications using the same. Further, the invention relates to image-forming apparatuses comprising high density foam rollers.
In a typical image forming apparatus, including but not limited to copiers, recorder, printers, and facsimile receptors, an image formed on a photoconductive image-bearing medium is developed by an image-developing device into a visible image by transfer of a toner (developer) to selected local spots on the imagewise exposed image bearing medium. The image developing device typically comprises a toner-containing case, an image developing roll, and a toner adder roll. A toner adder roll, also referred to as a toner supply roll, is typically an elastic roll adapted to supply the toner to the developing roll, which then transfers the toner to the image-bearing medium. The toner adder roll must be capable both of supplying a suitably controlled amount of the toner to the developing roll, and scrubbing off unused toner from the developing roll, so that the toner is uniformly distributed on the developing roll. The toner adder roll and developer roll have the same rotational direction with respect to one another and there is typically a nip at the contact area between the toner roll and the developing roll. Hence, the toner roll and developing roll are moving in opposite directions at the nip in order to effect the scrubbing and toner-supply functions.
Several features of the toner adder roll are important to its proper functioning in the dual capacity of both application and removal of toner. Scrubbing ability is enhanced by increasing hardness, contact pressure at the nip, and surface roughness. Excessive hardness, however, may lead to undesirable accelerated wearing of the developer roll, deterioration due to grinding of the toner particles, and excessive electrostatic charging of the toner. This eventually results in deterioration in the quality of the reproduced image.
Typically, then, toner adder rolls are formed of flexible polymeric foams. However, such foams exhibit inherently low electrical conductivity, and, therefore, resistivity and static discharge must be controlled via conductive agents incorporated in the roll. Open cell, reticulated foams have historically been preferred for toner adder rolls because they exhibit the necessary flexibility and surface roughness, and can easily be made conductive by exposing the foam to aqueous dispersions comprised of suitably conductive agents such as carbon black, and a resin binder. Described in greater detail below, an open-cell structure is generally defined as one in which the cells communicate with one another. A reticulated cell structure represents the extreme form of open-cell structures, and formation requires an additional step comprising subjecting the foam to high pressure and temperature, or by a chemical process, such that the cell membranes between the cells are eliminated, and all that remains is a cellular skeletal matrix. The reticulated cell structure, disclosed in U.S. Pat. No. 4,985,467, to Kelly et al., has been considered ideal for toner adder rolls because it is highly permeable to liquid-agent dispersions, and readily allows the dispersion to penetrate and thereby coat the internal foam structure.
One well-known problem with many conventional image-developing devices arises from gradual hardening of the toner adder roll. In a developing device using a toner adder roll made of a reticulated foam material, toner enters the cells of the foam material through the surface cells on the peripheral portion of the adder roller which contacts the developing roll. Large cellular size, lack of cellular membranes and ease of inter-cellular flow results in gradual accumulation of toner in the interior of the roll, hardening the roll excessively over time. As a result of the hardening, the contact pressure of the adder roll to the developing roll increases. Accordingly, the toner fuses on the developing sleeve, deteriorates, and the driving torques for the developing sleeve and the toner adder roll increase. The hardening is not uniform in the longitudinal direction of the adder roll, and, therefore, toner is not uniformly supplied to or scrubbed from the developing roll. Hence, the triboelectric charge of the toner on the developing roll is non-uniform, and the toner layer thickness on the roll is non-uniform.
These types of foam have proven satisfactory for monochrome print applications. However, it has been found that the use of such foams in color applications contributes to significant print variation problems, specifically a problem known in the printer industry as xe2x80x9cfade to colorxe2x80x9d wherein there is a band of light print at the top of a page of solid area print with a width corresponding to the first revolution of the developer roll.
Toner-related problems are generally exacerbated when color toner is used. Color toner typically has fluidity inferior to that of black toner. Specifically, color toner contains a resin of a type having a multiplicity of low-molecular weight components in order to realize color transmissivity and a dispersant for uniformly dispersing color pigment. An electrophotographic process using color toner typically comprises four developing devices, which would require a substantial increase in the size of the image-forming apparatus over that forming a monochrome image. To keep the size of the apparatus consistent with that required by monochrome processes, the size of the developing device is decreased, and the density of pigment in each toner particle is typically increased to produce a required image density with a smaller quantity of toner, hence allowing the capacity of the toner case to be reduced. This increase in the pigment component also deteriorates the fluidity of the toner, causing a greater rotational load, changing the torque, with resulting jitters in the formed image. Increasing the pigment component also raises the surface area ratio of the pigment component on the toner particle surface in general. Toner generally has a certain polarity and is frictionally charged by an electrified member having a polarity opposite to the polarity of the toner (such as the developing roll), so that the charging of toner is stabilized. When the toner particles have a large surface area of pigment, the electrified members, such as the developer roll, encounter filming attributable to mechanical contact and sliding. The charged characteristic of toner deteriorates and becomes unstable.
Conventional toner tends to have a relatively small particle size and relatively low melting point, so as to meet demands for improved image quality during increased speed of printing. Toner of this size tends to aggregate due to electrostatic charging, and the aggregate masses can form films of toner on the outer circumferential surface of the developing roll, so that the remaining toner films cannot be sufficiently scrubbed off by the toner adder roll, leading to occurrence of undesirable variation of the toner concentration or density of the reproduced image.
In order to prevent the clogging and hardening of toner within the toner adder roll, several solutions have been proposed. For example, toner adder rolls with non-cellular skin layers for preventing the penetration of toner have been provided. However, this solution often results in an undesirable fog due to fusing of the toner particles to the developing roll surface caused by excessive contact between the toner adder roll and developing roll. Also, toner adder rolls comprised of higher density inflexible foams with open but smaller cells may result in decreased or nonexistent entry of toner into the roller, but, typically, the overall hardness of the roll is too high, negatively impacting scrubbing ability, and, like the skin-layered rolls, the greater contact surface between the toner adder roll and developing roll led to frictional fusing of the toner to the developer roll.
Toner adder rolls formed from closed cell foams have been disclosed, and hardening of the roll due to collection of the toner in the pores can be prevented. However, such foams typically have higher density, and, if the density of the roll is too high, it has been found that there arises the same problems as when the roll of the open-cell foamed material is hardened. In addition, the contact surface with the developer roll is undesirably increased such that frictional fusing of toner is likely to occur, as with the smaller-celled inflexible foams. The foam must be flexible enough to form a nip at the interface with the developing roll that is effective to scrub unused toner, and resilient enough to be deformed by compression at the nip, yet xe2x80x9cspring backxe2x80x9d to substantially its original diameter upon rotating past the nip so that effective engagement at the scrubbing side of the nip is maintained.
Toner adder rolls comprised of foams wherein an additional precision-foam contouring step is employed are known. The contouring step is often employed in order to provide the roll with an outer peripheral surface sufficiently irregular to provide the requisite scrubbing and avoid the fusing of toner associated with excessive contact between the toner adder roll and developing member. These rolls typically comprise periodic surface irregularities such as trapezoidal or helical protrusions. These rolls, however, are more expensive to manufacture. Further, they yield inconsistent scrubbing results and inconsistent triboelectric charging of the toner due to the roll diameter variance created by the presence of the protrusions.
Thus, it would be advantageous to provide a toner adder roll which overcomes the disadvantages of the prior art while providing features that confer functional requirements and advantages. Also, it would be advantageous to provide a toner adder roll with a surface effective both to supply and scrub toner and avoid excessive contact force at the nip. Finally, it would be advantageous to provide a toner adder roll specifically adapted to functioning with colored toner in multi-chrome image-forming applications, and to do so in a cost effective manner.
Accordingly, it is an object of the present invention to provide higher density foam rollers suitable for use, for example, in image-forming apparatuses. It is a further object to provide such rollers which overcome disadvantages of the prior art.
In one embodiment, the invention is directed to a roll for use in an image-forming apparatus comprising polymeric foam exhibiting a density of at least about 6 pounds per cubic foot (pcf), and a 25% compression force deflection (CFD) of at least about 2.5 pounds per square inch (psi).
In another more specific embodiment, the invention is directed to a toner adder roll for use in an electrophotographic image-forming apparatus comprising a substantially homogeneous layer of open-celled, non-reticulated polyurethane foam cylindrically disposed about a metal shaft. At least one conductive agent is dispersed throughout the foam and the foam exhibits a density of at least about 6 pcf, a 25% CFD of at least 2.5 psi, an average linear cell count of from about 90 to about 120 pores per inch (ppi), and a resistivity of less than about 1xc3x97109 ohm-cm.
The present invention is also directed to image-forming apparatuses, and, more specifically, to an electrophotographic image-forming apparatus, comprising the aforementioned rolls.
Further, the invention is directed to a method for applying toner to a developing member in an electrophotographic image-forming apparatus comprising applying the toner via a roll comprising polymeric foam wherein the foam exhibits a density of at least about 6 pcf and a 25% compression force deflection of at least about 2.5 psi.
The rolls, image-forming apparatuses and methods of the invention advantageously employ rolls which may be easily manufactured by methods well-known in the arts, and provide precise toner application in a relatively inexpensive manner. These, and additional objects, embodiments and advantages are disclosed in further detail in the following detailed description.